1. Field of the Invention
This invention relates to methods of and apparatus for electrodeless discharge excitation. Accordingly, it is a general object of this invention to provide new and improved methods and apparatus of such character.
2. Description of the Prior Art
The following U.S. patents, assigned to the assignee of this application, relate to helical structures used with electrodeless lamps.
______________________________________ Patent No. Patentee Issue Date ______________________________________ 3,942,058 Haugsjaa et al. March 2, 1976 3,942,068 Haugsjaa et al. March 2, 1976 3,943,404 McNeill et al. March 9, 1976 ______________________________________
The patentees of U.S. Pat. Nos. 3,942,068 and 3,943,404 correspond to the applicants of this application.
U.S. Pat. No. 3,942,058
U.S. Pat. No. 3,942,058 to Haugsjaa et al., entitled "Electrodeless Light Source Having Improved Arc Shaping Capability", discusses the manner of operation of the electric fields at column 4, lines 26-44, with regard to region I(R.sub.I) where the lamp is inside the helix, as follows:
A helical line inside a conducting cylinder constitutes a slow wave structure; if .PSI. is the pitch angle, such that cot .PSI.=2.pi.a/p, where a is the helix radius and p is the pitch, then the wave propagation velocity is v=c sin .PSI.. Thus, the phase velocity is always less than the velocity of light. The wavelength along the helix is reduced, .lambda..sub.H =.lambda..sub.o sin .PSI.. Hence, a quarter-wave termination fixture can be reduced in length by the factor sin .PSI.. PA1 Wave propagation on a helix is, in general, complex. However, much of the observed behavior of arcs in helices can be understood in terms of the dominant mode. This mode has a field pattern with an electric field component E.sub.z in axial direction, and also a field E.sub..PHI. in the azimuthal direction. Thus, in FIG. 2 a lamp placed in the region I(R.sub.1) inside the helix might have either an axial arc or a toroidal (donut shaped) arc lying the horizontal plane. The ratio of the fields in (sic) controlled by the helix parameters; E.sub.z /E.sub..PHI. =(a/r) cot .PSI.. PA1 A lamp placed just above the helix in region II (R.sub.11) would be excited in the axial direction. PA1 Electrodeless lamps have the potential for extremely long life, because there is no need for the arc discharge to be in contact with any material, either electrodes (i.e., since there are none) or the lamp envelope. However, there is a tendency in the operation of high pressure electrodeless lamps in termination fixtures for the arc to be in close contact with the envelope walls, thereby causing damage to the wall and foreshortening the lamp's lifetime. Typically, this attachment of the arc to the wall of the lamp occurs at the point where the lamp is in contact with the center conductor of the fixture, where the electric field intensity is high. PA1 The purpose of this invention is to provide an electrodeless arc discharge in which the arc is sufficiently isolated from the wall of the lamp envelope so that no damage to the wall occurs over a long period of time. An arc may be isolated from a particular area by adjusting the detailed power balance for that area. This involves the equation P.sub.e -P.sub.h =P.sub.r, where P.sub.e is the power gained electrically, P.sub.r is the power radiated and P.sub.h is the power lost as heat. PA1 Generally, an arc exists in a region where P.sub.e &gt;P.sub.h. This invention relates to a way in which P.sub.e can be made small enough in a region so as not to allow an arc to exist there. P.sub.e =n.sub.e .mu.E.sup.2, where n.sub.e is the electron density, .mu. the electron mobility and E the electric field strength. PA1 In a fixture such as the termination fixture described herein, it is possible to adjust the field strength E by one of several techniques. These techniques included adjusting the position of fixture conductors, shaping the center conductor or field coupling probe, adjusting the position and shape of lossless magnetic or dielectric material within the fixture, adjusting the shape of the exterior walls of the fixture, adjusting the configuration, position and material of the lamp envelope, and controlling the current paths on the exterior fixture walls by use of a pattern of conductors. Therefore, by using one or several of these techniques, one may reduce the field strength near the lamp wall, and thus the arc can be isolated from the walls. With arc isolation from the envelope wall, the lifetime is increased several orders of magnitude. PA1 The presently preferred way of arc shaping is by appropriately shaping the geometry of the end of the inner conductor. In one preferred embodiment, the inner conductor of the termination fixture is shaped in the form of a helix. A helical center conductor allows use of a shorter termination fixture than a quarter wavelength; it allows for control over the field shape so that, the arc can be isolated from the envelope and, finally, it provides a means for impedance matching the lamp to the input. PA1 Wave propagation on a helix is, in general, complex. However, much of the observed behavior of arcs in helices can be understood in terms of the dominant mode. This mode has a field pattern with an electric field component E.sub.z in axial direction, and also a field E.sub..PHI. in the azimuthal direction. Thus, in FIG. 2 a lamp placed in the region I (R.sub.I) inside the helix might have either an axial arc or a toroidal (donut shaped) arc lying the horizontal plane. The ratio of the fields in (sic) controlled by the helix parameters; E.sub.z /E.sub..PHI. =(a/r) cot .PSI.. A lamp placed just above the helix in region II (R.sub.II) would be excited in the axial direction. PA1 the pitch of the slot 42 is variable to create a strong axial field in a region within the inner conductor and to compensate for at least a part of the reactive impedance of the lamp during excitation. PA1 Axial arcs, toroidal arcs, and discharges apparently excited by both axial and azimuthal fields are obtainable. In particular, a lamp filled with mercury, sodium iodide, scandium iodide and argon having a cylindrical envelope was run axially; the arc was observed to isolate from both ends of the envelope when the lamp was appropriately placed just inside the helix in region R.sub.I. Small mercury lamps in both cylindrical and spherical envelopes, have been run with isolated toroidal discharges, excited by the azimuthal field inside the helix. Finally, a metal halide lamp in a spherical envelope run inside the helix was apparently coupled to both components of the dominant helical mode. The discharge is quite diffuse and detached from the lamp wall. PA1 The light source includes a termination fixture which is coupled to a source of high frequency power. The fixture has an inner conductor and an outer conductor disposed around the inner conductor. These conductors have lengths of a quarter wavelength and cross-sectional dimensions selected to produce a fixture characteristic impedance which matches the real component of the complex impedance of an electrodeless lamp, which forms a termination to the conductors, to the impedance of the coupled source. The fixture includes a reactive impedance device which is coupled between the outer end of the inner conductor and the lamp. This device compensates for the reactive component of the complex impedance of the termination when the lamp is in the excited state. Usually, the reactive component of the lamp impedance is capacitive so that the compensating device is an inductance in series between the inner conductor and the lamp. Preferably, the inductance is a helical coil. PA1 In the present invention, it is possible to obtain a perfect impedance match, despite the load impedance being complex. The compensating coil reduces fixture losses since the largest standing waves are confined to the lamp-coupler region. Also, the coil is a high thermal resistance element by virtue of its length and cross-section, thus reducing heat conduction losses from the lamp. PA1 Since the dominant reactive impedance of the lamp is usually capacitive, the device 40 preferably is inductive. The inductive device 40, as illustrated in FIG. 5, is preferably a helical coil having a first end 42 in contact with the end of the inner conductor 20 and a second end 44 in contact with the lamp 14. In FIG. 5, coil 40 is formed with a bend 46 such that the first end 42 may be positioned in a receiving opening 48 formed in the end of the inner conductor 20. This provides a useful technique for holding the coil in a stationery (sic) position with respect to the inner conductor. The second end of the coil may be formed with a spherically shaped element to avoid high field breakdown. PA1 The helical coil overcomes the imperfect coupling by providing a compensating series inductance. This is done by the use of a short helical extension to the center conductor of the termination fixture. PA1 That is, the capacitance of the lamp is exactly compensated by the coil inductance, and the high frequency source impedance may be matched to this termination by the use of a quarter wave termination fixture. There are several advantages to this scheme of coupling. First, this embodiment makes it possible to run lamps with complex impedances quite efficiently. The coil reduces fixture losses since the largest standing waves are confined to the lamp coupler region and it provides a high thermal resistance element by virtue of its length and cross-section, thus reducing heat conduction losses from the lamp. PA1 The primary purpose of the coil at the lamp is to provide some impedance matching for the lamp in the excited state.
The sentence following discusses the direction of excitation when the lamp is placed just above the helix:
U.S. Pat. No. 3,942,068
U.S. Pat. No. 3,942,068 suggests the use of a helix to reduce the microwave field at the base of an electrodeless lamp, and thereby reduce the arc attachment to one end of the lamp envelope.
As set forth in the Abstract, the end of an inner conductor is shaped such as to inhibit the arc of the lamp from attaching to the lamp envelope. In one embodiment, the inner conductor end is shaped as a hollow helical coil which generates an axial and azimuthal electric field component to create a toroidal arc within the lamp.
As set forth in the Summary of the Invention, some objects of the invention were to provide an electrodeless light source in which the arc in an electrodeless lamp is not attached to the wall of the lamp envelope, and to provide a termination fixture for the lamp in which the inner conductor is shaped such as to control the electric field strength at the lamp envelope wall. The invention relates to a light source which includes a source of power at a high frequency, an electrodeless lamp having an envelope made of a light transmitting substance and a volatile fill material enclosed within the envelope, the fill material emitting light upon breakdown and excitation, a termination fixture having an inner conductor and an outer conductor disposed around the inner conductor, the conductors having one end which couples power to the lamp and another end which is coupled to the source and the inner conductor having means at the lamp coupling end for controlling the electric field strength in a region adjacent to the interior wall of the envelope to inhibit the formation of an arc within the region. In one embodiment, the field controlling means is an inner conductor shaped as a hollow member having a slot formed therein to form a helical arrangement, the electric field having both an axial and an azimuthal component.
As discussed in the General Operational Description:
As set forth in the Description of the Preferred Embodiments, preferably,
U.S. Pat. No. 3,943,404
U.S. Pat. No. 3,943,404 suggests the use of a helix as a means for matching the complex impedance of the electrodeless lamp to the impedance of the source. As stated in the Abstract, a helical coil couples the end of the inner conductor to the electrodeless lamp. The purpose of the coil is to make the impedance of the lamp, as viewed, electrically, from the end of the inner conductor appear as having only the real component. The quarter wave fixture then matches the real impedance to the source impedance.
As set forth in the Summary of the Invention:
And in the Description of Preferred Embodiments (columns 4-5):
In both the foregoing U.S. Patents, the electrodeless lamp lays mostly outside of the helix.
Other Patents
The following U.S. Patents may be of interest. At least one of the patentees of each patent is an applicant of this application. All patents have been assigned to a common assignee.
______________________________________ U.S. Pat. No. Patentee Issue Date ______________________________________ 3,943,401 Haugsjaa et al. March 9, 1976 3,943,402 Haugsjaa et al. March 9, 1976 3,943,403 Haugsjaa et al. March 9, 1976 3,993,927 Haugsjaa et al. November 23, 1976 3,995,195 Haugsjaa et al. November 30, 1976 3,997,816 Haugsjaa et al. December 14, 1976 4,001,631 McNeill et al. January 4, 1977 4,001,632 Haugsjaa et al. January 4, 1977 4,002,943 Regan et al. January 11, 1977 4,002,944 McNeill et al. January 11, 1977 4,041,352 McNeill et al. August 9, 1977 4,053,814 Regan et al. October 11, 1977 4,065,701 Haugsjaa et al. December 27, 1977 4,070,603 Regan et al. January 24, 1978 ______________________________________
Also of interest is the following U.S. Patent which relates to electrodeless lamps.
______________________________________ 3,787,705 Bolin et al. January 22, 1974 ______________________________________
Prior Art Statement
The U.S. patents set forth hereinabove constitute prior art which includes, in the opinion of the applicants and their attorney, the closest prior art of which they are aware. This prior art statement shall not be construed as a representation that a search has been made or that no better art exists.